Soluble and Plant Available Phosphorus in Acid Soils

Sammanfattning: The P availability and solubility differ between soils and plants seem to have different ways of utilising the P present. The many P fractions present in soil interact and are closely related to another and transformations may easily occur. Due to this more P than the soluble fraction is available to plants. Different plant species utilise the P in soil differently; the relationship between the plant P concentration and the P content of the soil differs among species. More knowledge about plant responses to low P concentrations in soil, and on the replenishment rate of the soluble P pool is needed. Different processes and reactions might also be activated as P status changes, or if the major P pool changes. However, little is known in this field of research. To determine P solubility in a particular soil, P sorption, pH, P content, distribution of P, and organic matter composition need to be investigated. Another consequence of the close relationship between the P fractions in the soil is that added P is distributed over the soil P fractions and increases the soils overall P status including the most easily available P fractions. The effect of a P fertilisation can be detected long after fertilisation has ceased as increased easily extractable P determined with methods that is considered available to plants. In contrast addition of compounds that increase one particular P fraction of the soil, for example Ca, decreases the soluble P pool in the soil. If this build-up of Ca-phosphates decreases the potentially available fraction is however not certain, this depends on if the fraction formed is in equilibrium with the directly available, more soluble P.